1. Field of the Invention
The present invention relates to a variable shape mirror and its manufacturing method, and more particularly to a small variable shape mirror applying the semiconductor technology and its manufacturing method, in a variable shape mirror capable of varying the curvature continuously.
2. Description of the Related Art
In a micro optical system applied in photo pickup or other micro optics, hitherto, for the purpose of simplifying the mechanism relating to focusing by using electromagnetic actuator, an ultrasmall variable focus mirror capable of varying the curvature of the reflection plane has been proposed.
In a small photographic optical system, application of variable focus mirror contributes to reduction of size.
Such variable focus mirror is expected to be manufactured at low cost and high precision by applying the so-called MEMS (Micro Electro-Mechanical System) based on the semiconductor manufacturing technology.
As an example of such technology, a reflecting mirror device as a variable focus mirror is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-101402.
The reflecting mirror device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-101402 is explained briefly by referring to FIGS. 8A and 8B, and FIGS. 9A to 9E.
FIGS. 8A and 8B are sectional view and perspective view showing the configuration of the reflecting mirror device of electrostatic attraction driving system disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-101402.
In FIGS. 8A and 8B, reference numeral 11 is a glass or other insulating substrate. (hereinafter called glass substrate), and a fixed side electrode layer 12 of conductive thin film is applied on the top of the glass substrate 11.
Reference numeral 13 is a silicon or other semiconductor substrate (hereinafter called silicon substrate), and a silicon dioxide thin film 14 is formed as a insulating film on a principal plane of the silicon substrate 13.
Reference numeral 15 is a vacancy formed on other principal plane in the central part of the silicon substrate 13, and this vacancy 15 is to set the central part of the silicon dioxide thin film 14 displaceably in the thickness direction.
Reference numeral 16 is a movable side electrode layer, and this variable side electrode layer 16 is laminated on the thin silicon dioxide film 14.
The central parts of the silicon dioxide thin film 14 and movable side electrode layer 16 form a reflecting mirror section 17.
The reflecting mirror section 17 is recessed and deformed to the fixed side electrode layer 12 side by the voltage applied both electrode layers of the fixed side electrode layer 12 and movable side electrode layer 16.
The silicon substrate 13 is bonded to the glass substrate 11 by way of a spacer member 18, with the silicon dioxide thin film 14 side downward.
Also, in FIGS. 8A and 8B, reference numeral 19 is a silicon dioxide thin film formed on other principal plane of the silicon substrate 13.
This reflecting mirror device is manufactured according to the manufacturing process diagrams shown in FIGS. 9A to 9E.
FIGS. 9A to 9E are sectional views for explaining the manufacturing process of the reflecting mirror device of electrostatic attraction driving system disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-101402.
First, as shown in FIG. 9A, silicon dioxide thin films 19 and 14 of 400 to 500 nm in thickness are formed on both sides of a silicon substrate 13 of plane azimuth <100> of which both sides are polished to mirror smoothness.
A gold thin film 16 of about 100 nm in thickness is applied on the silicon dioxide thin film 14 of the lower side.
Next, as shown in FIG. 9B, a photo resist 20 of specified pattern is applied on the silicon dioxide thin film 19, and a circular window opening 21 is formed by photolithography.
With the lower side of the substrate in protected state, a window is opened in the silicon dioxide thin film 14 by a hydrofluoric acid solution, using the photo resist 20 as mask.
Further, as shown in FIG. 9C, the silicon substrate 13 is immersed in an aqueous solution of ethylene diamine pyrocatechol, and the silicon substrate is etched from the area of the window opening 21.
At this time, as shown in the drawing, etching is stopped when the silicon dioxide 16 at the lower side is exposed.
Thus, a thin film of reflecting mirror section 17 composed of silicon dioxide film 14 and gold thin film 16 is left over.
On the other hand, in other process than mentioned above, as shown in FIG. 9D, a metal film of 100 nm in thickness is formed as a fixed side electrode layer 12 on the top of a glass substrate 11 of 300 nm in thickness.
Then, as shown in FIG. 9E, a silicon substrate 13 is adhered on the glass substrate 11 by way of a polyethylene spacer member 18 of about 100 μm in thickness, so that a reflecting mirror device is manufactured as shown in FIGS. 8A and 8B.
This variable focus mirror manufactured by adhering substrates as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-101402 involves the following first and second problems when applied in the optical system where a high focusing performance is required such as a high definition camera.
The first problem is about the opening shape of the upper substrate on which the reflecting plane is formed.
That is, to form an opening, it is most preferable to etch by using an alkaline solution such as aqueous solution of ethylene diamine pyrocatechol mentioned above or potassium hydroxide from the viewpoint of cost and combination with thin film members.
By etching, however, due to crystal azimuth dependence of the silicon substrate, an accurate circular or elliptical opening shape cannot be obtained.
If the opening is square or polygonal, the deformation of the reflecting plane due to stress is asymmetrical, and the astigmatism increases, and the focusing performance is lowered.
The second problem is distortion of the upper substrate in the assembling process.
That is, the upper substrate is a single crystal silicon substrate, and a high mirror flatness is achieved, but when bonding with the lower substrate, or due to stress caused in the connection process for leading out the electrode of the upper substrate to the external lead, the upper substrate is slightly deformed, and an adverse effect is caused on the mirror focusing performance.
This problem may be somewhat avoided by keeping the junction position of the substrates or the connection position of the electrode of the upper substrate to the external part sufficiently away from the mirror opening area, but, as a result, the entire size of the mirror element is increased, which is contradictory to requirements of smaller size and lower cost of the optical system.
Incidentally, as the driving method of this kind of variable shape mirror, aside from the method of using electrostatic attraction force disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-101402, a method of using piezoelectric effect disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 3-81132, and a method of using fluid pressure disclosed in Jpn. Pat. Appln. KOKAI Publication No. 1-219801 are known.
These methods have their own merits and demerits, but the method of using fluid pressure is advantageous in an application where very high response is not required but a relatively large displacement is needed from the concave surface to the convex surface.
As an example of a variable shape mirror of such fluid pressure drive, the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 1-219801 is briefly explained by referring to FIG. 10.
This variable focus mirror 1 is composed of a shell 2, chamber pressure adjusting device 3, and a reflecting mirror 4.
A pressure chamber 5 is formed in the shell 2, and a holder 7 for holding the reflecting mirror 4 airtight by O-rings 6 is formed in its opening.
In the pressure chamber 5, a pressure gauge 8 and a piping 9 of the chamber pressure adjusting device 3 are connected.
The piping 9 is composed of a compressor piping system 9a and a vacuum pump piping system 9b, and which are changed over appropriately between a compressor 21a and a vacuum pump 21b by means of electromagnetic operation valves 10a, 10b. 
To change over, the electromagnetic operation valves 10a, 10b are opened or closed by a controller 22.
The reflecting mirror 4 is made of a thin plate, and its reflecting plane 23 is coated with a reflecting material such as aluminum.
In the variable focus mirror 1 having such configuration, to form a concave reflecting plane 23a, the controller 22 is operated to close the electromagnetic operation valve 10a and open the electromagnetic operation valve 10b. 
As a result, the pressure chamber 5 communicates with the vacuum pump piping system 9b, and is evacuated to a negative pressure by the vacuum pump 21b. 
In this state, therefore, the reflecting mirror 4 is deflected to the side of the pressure chamber 5, and a concave reflecting plane 23a is formed.
On the other hand, to form a convex reflecting plane 23b, the controller 22 is operated to open the electromagnetic operation valve 10a and close the electromagnetic operation valve 10b. 
As a result, the pressure chamber 5 communicates with the compressor piping system 9a, and is compressed to a positive pressure by the compressor 21a. 
In this state, therefore, the reflecting mirror 4 is deflected to the opposite side of the pressure chamber 5, and a convex reflecting plane 23b is formed.
Further, by controlling the pressure in the pressure chamber 5 to be equal to the atmospheric pressure, the reflecting plane 23 maintains a flat reflecting plane 23c by its own elasticity.
The shape of the reflecting plane 23 can be varied by controlling the operation of the controller 22 according to the measurement of the pressure gauge 8, and the reflecting mirror 4 can be continuously set to an arbitrary focal length.
The variable shape mirror using such fluid pressure as driving source is particularly suitable to the application where change of focal distance in a wide range is required, as compared with the electrostatic attraction driving system in which the displacement is limited by the distance between electrodes or the piezoelectric driving system which is difficult to give a large deflection due to limit in the material of the reflecting plane.
The problem of the variable shape mirror using such fluid pressure as driving source is that it is difficult to reduce in size because pump or compressor is needed.
However, owing to the recent progress in micro machine technology, ultrasmall pumps applying the semiconductor manufacturing technology have been developed, and by using them, it is expected to realize a variable shape mirror of fluid pressure type to be assembled in a small device.
Nevertheless, the ultrasmall pump formed by the micro machine technology, generally, cannot generate a large pressure difference in a short time, and it is required to form the thin film as the reflecting plane by using a material of a very small rigidity, so that a large displacement may be obtained by a small pressure difference.
In this case, for precise control of displacement, pressure measurement of a very high resolution is needed, but if a pressure measuring instrument having such high precision, a third problem is caused, that is, the size cannot be reduced and the cost is increased.